A process for preparing a dialkyl carbonate is described in U.S. Pat. No. 5,359,118. The patent discloses a process in which di(C1-C4 alkyl) carbonates are prepared by transesterification of an alkylene carbonate with a C1-C4 alkanol. The alkylene carbonate and an alkanol feedstock are reacted countercurrently in a column with the aid of a catalyst. The catalyst is usually homogeneous, although the use of heterogeneous catalysts is also suggested. The alkylene carbonate is introduced into the upper part of the column and trickles down from above. The alkanol feedstock comprising a pure alkanol and a stream, comprising the alkanol and also the dialkyl carbonate, is fed into the column at a lower part. The alkanol flows upward and reacts countercurrently with the alkylene carbonate to obtain dialkyl carbonate with unreacted alkanol as the top effluent and the alkanediol with any entrained alkanol as the bottom effluent. The top effluent is subjected to distillation treatments to yield an alkanol-rich stream comprising the alkanol and minor amounts of the dialkyl carbonate. This stream is fed to the column as part of the alkanol feedstock. The bottom stream is worked-up resulting in an alkylene glycol stream and a catalyst-containing concentrate that is partly recycled.
The patent discloses the formation of high-boiling by-products, such as polyglycols. In the process these high-boiling by-products are contained in the catalyst-containing concentrate. Part of the concentrate is recycled to the transesterification, whereas another part is discarded.
Although the process discloses the formation of polyglycols, it does not address the problem of the removal of such polyglycols, in particular the dimers of the alkanediol, also known as dialkylene glycols. Moreover, the process presumes that no alkylene carbonate leaves the reactor unconverted. In practice, the transesterification to dialkyl carbonate will not be 100%. Therefore, the bottom product of the process will contain not only polyglycols, as suggested in U.S. Pat. No. 5,359,118, but also some unconverted alkylene carbonate. In view of their boiling points it is very difficult to separate dialkylene glycols from the corresponding alkylene carbonates. This has not been acknowledged in the process according to U.S. Pat. No. 5,359,118. Therefore the bottom product contains contaminants that bear the risk of building up during continuous recycle.
It is an object of the present invention to overcome this drawback.